Downlights are lighting fixtures typically mounted in ceilings for illumination directly below them. Conventionally, these ubiquitous luminaires generally comprise an incandescent spotlight mounted within a can. Since incandescent bulbs operate hot anyway, they are not thermally bothered by the can being a trap for hot air. It would be highly desirable to replace incandescent light bulbs with lamps using light-emitting diodes (LEDs), because LEDs last much longer and use much less electricity than incandescent bulbs. However, even LEDs produce a significant amount of heat (about ¾ of their electrical power consumption) and LEDs are temperature-vulnerable, so that downlights are a more difficult lighting application than anticipated. This is because the optics of a conventional downlight dictate that the actual light source be installed at the top of the can, facing downwards. The waste heat of the LEDs cannot very effectively be dissipated passively into the stagnant hot air of the typical downlight can. This poor heat dissipation typically limits the total electrical power that can be handled in a solid state LED downlight to a maximum of approximately 5 watts.
This power limit can only be overcome if the can is dramatically increased in size to aid in heat management, or if active cooling or ventilation is provided, a severe limitation. Furthermore, the best commercially available 5 watt LED sources have an efficacy of 60 lumens per Watt (LPW), including driver losses. This limits such a solid-state downlight to a flux of approximately 300 lumens. It would be desirable to have a standard size solid state downlight producing 600 to 1000 lumens. That light output is achievable only if the heat management can handle a minimum of 10 watts, which mandates moving the LEDs down from the top of the can.
It is possible to install LEDs at the luminous aperture of the downlight, facing upwards. This is in some respects similar to an entirely traditional arrangement of a light-bulb within a reflector. However, when a solid-state light source with hemispheric emission is substituted for an incandescent lamp with almost spherical emission, the reflector dish does not have to be as deep. Also, the LED is mounted facing upward, so that its entire light output is directed into the reflector, where the incandescent bulb is usually mounted with its base upward, so that its central rays are emitted directly downwards, and the base forms a dead area at the center of the reflector.
As an example, FIG. 1 shows an LED downlight 100, comprising downward-facing reflector dish 101 and upward-facing LED light source in the form of light engine 110. Light engine 110 in turn comprises chip array 111, dome 112, package 113, circuit board 114, and heat-sink 120. The exact shape of reflector dish 101 would be dictated by the desired width and prescription of its output pattern.
It is further possible to utilize another employment of light-emitting diodes, within an integrating cavity, the exit aperture of which would shine upward into a downward-facing reflector dish. FIG. 2 shows another example of an LED downlight 200, comprising downward-facing reflector dish 201 and upward-facing light engine 210. Light engine 210 in turn comprises multiple chip arrays 211 positioned within integrating box 212, the light output of which shines upwards out through exit aperture 213. The interior walls of box 212 are diffusely reflective, as with white paint. They also would be thermally conductive, conveying heat from the chips 211 to finned heat sink 220. Exit aperture 213 can include a diffuser, for color mixing in the case of chip arrays 211 being of different colors, such as red, green, and blue.
Thirdly, it is possible to utilize a remote phosphor light source shining upwards into a downward-facing reflector dish. FIG. 3 shows a further example of an LED downlight 300, comprising downward-facing reflector dish 301 and upward-facing light engine 310. Light engine 310 comprises multiple chip arrays 311 positioned within integrating box 312. All of the chips are blue-emitting, however, in order to stimulate transparent-mode remote phosphor 313, which covers the outlet of integrating box 312. Most of the blue light striking phosphor 313 will be converted to yellow light, and the remaining blue light combines with the yellow converted light to make a balanced white output. Yellow light, as well as reflected blue light, will also shine back within the integrating box 312 and be recycled back upon the remote phosphor 313. Optionally, the multi-chip array can include one or more secondary red or other color LEDs (including secondary blue LEDs of a different wavelength blue from the primary blue LEDs) so that the light source can produce a range of color temperatures. By adjusting the output of these secondary LEDs, either when the light source is manufactured or in operation, the source can, for example, emulate the light from an incandescent lamp (typically from 2700 to 3000K color temperature) or a cool white light source.
FIG. 3 also shows two items that may be present in the luminaires 100, 200, but are omitted from FIGS. 1 and 2 for the sake of clarity. Exemplary structural support vane 330 keeps light source 310 in position and provides a conduit for wiring or the like for power supply and electronic control. In a typical practical construction, two or more vanes are used to produce a structurally stable configuration without requiring a very stiff, bulky vane. Exemplary ray 340 proceeds from remote phosphor 313 and is reflected downward to subtend output angle 341 with the vertical. In general, the shape of the reflector dish is adapted for a particular desired distribution of such output angles for all rays from the light source. This distribution in turn depends upon the downlight's illumination goals, which typically are uniform illumination over a nearby plane, such as a floor or table.
These configurations are disclosed as examples of previously-proposed designs for LED downlights. They suffer from two problems, however. The light source and its heat sink project well below the reflector dish, an aesthetic problem which can only be solved by positioning the entire assembly within a ceiling-can that will shield the heat sink from view (shown respectively as can 150 in FIG. 1, can 250 in FIG. 2, and can 350 in FIG. 3). This in turn will cause the accumulation within the can of hot air from the heat sink, leading to overheating of the LED chips within the light source. Also, using this approach the light source and its associated components typically block some of the light coming from the reflector. In such a previous design approximately 10% is lost. Finally, this system must employ a reflector that does not have a vent hole at its top (if it is not to incur a further drop in efficiency because of light lost through the vent hole). Such a vent hole is desirable if convective thermal heat transfer is to be maximized. The present invention makes it possible to solve, or at least mitigate, some or all of these problems with the introduction of a further optical element, a flux-redistribution lens.